The inventions relates to an apparatus for the measurement of fluid flow. More specifically, the invention relates to a mass airflow meter wherein the electrical properties of a resistive element vary in proportion to the mass of air flowing across the element.
Mass airflow meters containing resistive elements are known for providing an electrical measurement of the mass of air inducted into an internal combustion engine per unit of time. A resistive element, such as a resistive wire or a resistive film, is disposed within the airflow and electrically heated by passing electric current therethrough. The electric power applied to the resistive element is controlled to maintain a substantially constant element temperature regardless of airflow. Stated another way, the electric power applied to the resistive element is controlled to maintain a constant resistance as the heat transfer by convection to the mass airflow changes. Accordingly, a measurement of the electric power may be translated into a measurement of the airflow. However, there is also heat transfer from the resistive element by conduction to contiguous surfaces and also heat transfer by thermal radiation. Unless the heat transfer by conduction and radiation are both negligible, the measurement of airflow will be in error.
For mass airflow meters which utilize a resistive wire, heat transfer by conduction and radiation are generally minimal. In a typical hot wire airflow meter, a 70 micron platinum wire several inches in length is stretched between two support members. The portion or the wire in contact with the support members is sufficiently small with respect to the overall wire length such that heat transfer by conduction to the support members may be ignored. With respect to heat transfer by radiation, the small surface area and low emissivity of the wire result in negligible thermal radiation as compared to heat transfer by convection.
In the case of a hot resistive film such as a nickel foil deposited on a substrate, however, the conductive heat transfer between the film and substrate may be appreciable. Approaches have been utilized to heat the substrate and thereby reduce the conductive heat transfer. For example, see U.S. Pat. Nos. 4,214,478 and 4,283,944. Heat transfer by radiation, however, is generally not a problem in these approaches. Even though the surface area of resistive films is large compared to a wire, the low emissivity of the film results in minimal radiation as compared to the heat conducted between the film and the substrate.
Heat transfer by radiation has been recognized to be a problem in a configuration wherein the resistive film is coated or sandwiched between layers of either plastic or glass. In these flowmeters, thermal radiation cannot be ignored due to the relatively high emissivity of the coating and the relatively large surface area of the resistive film. For example, see SAE 860406, entitled "Mass Airflow Sensor: Ambient Temperature Compensation Design Considerations", by G. A. Gurtcheff and L. D. Hazelton. This paper proposes lowering the emissivity by covering the plastic coating with a shiny metal such as gold. A disadvantage of this approach, besides expense, is that dirt accumulation and erosion will eventually raise the emissivity of the coating thereby increasing thermal radiation.